So-called instant foods, such as instant noodles, can be made edible simply by pouring hot water into the container and allowing it to stand for a while, for example, 1 to 5 minutes. Because of this easy cooking process, they are very popular among unmarried persons, persons on night duty, persons living alone, young people, etc.
These instant foods, for example, noodles in a cup, are cooked according to the following steps. After removing a transparent shrink-wrapping film, a lug of the lid heat-sealed to the fringe of the cup is pulled up with the fingers to strip back the lid by about 1/3 or 1/2 thereof. Ingredients other than noodles, such as dry vegetables and powdered soup, taken from separate bags if so packaged, are placed on the noodles, and hot water is poured into the cup. Then, the stripped portion of the lid is again returned to its original position on the fringe, and the thus closed cup is allowed to stand for a prescribed time, e.g., 1 to 5 minutes, during which time the contents of the cup are cooked. During the cooking, a weight is put on the lid, or the end of the lid is fixed with adhesive tape so as to prevent the lid from curling upward thereby preventing heat dissipation and assuring thermal efficiency. After completion of the cooking, the lid is completely stripped off the cup for serving.
Containers to be used for instant foods of this type include cups made of resins having satisfactory heat retaining properties, such as those prepared by plug-assist vacuum forming or pressure forming of polystyrene paper (extruded expanded polystyrene sheet), high-impact polystyrene sheet, and nylon/polyethylene laminated sheet; those obtained by in-mold expansion of expandable polystyrene beads; and those obtained by injection molding of polypropylene.
Conventional lids for these containers are made from four-layered laminated sheet having heat-sealability as shown in FIG. 5, composed of fine paper having provided thereon a print 14 as base layer 2, adhesive layer 3 formed by extrusion coating of a molten resin film of low-density polyethylene, an ethylene-acrylic acid copolymer, etc. or by coating a urethane type or polyester type liquid adhesive (so-called anchoring agent), aluminum foil layer 4 having a thickness of from 6 to 15 .mu.m, and heat-sealing layer 5 formed by extrusion laminating a low-melting resin (melting point: 80.degree. to 135.degree. C.), e.g., low-density polyethylene and an ethylene-vinyl acetate copolymer, or by coating a solvent solution of such a resin.
However, when hot water is poured into the half-opened container, the fine paper is heated by hot steam through the aluminum layer to release its water content and shrinks, causing upward curling of the lid as illustrated in FIG. 6. In order to keep the lid in the right shape, it has been necessary to put a weight on the lid after adding the hot water.
JP-A-U-58-133574 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application") discloses a lid comprising a paper or aluminum base having laminated thereon a transparent biaxially stretched polyethylene film or a biaxially stretched nylon film, which closes the half-opened container by itself through a bimetal effect caused by the steam.
However, the proposed lid proved to undergo inward curling due to excessively high thermal shrinkage of the biaxially stretched film as illustrated in FIG. 7, resulting in failure to tightly close the container.